Problem: $ B = \left[\begin{array}{rrr}2 & 1 & 1 \\ 1 & 1 & 0 \\ 0 & 1 & 0\end{array}\right]$ What is $ B^{-1}$ ?
Explanation: $ = \left[\begin{array}{rrr}0 & 1 & -1 \\ 0 & 0 & 1 \\ 1 & -2 & 1\end{array}\right]$